Detectors of ionizing radiation for creating an image of a scanned object, namely detectors of X-rays, gamma rays, beta rays or other ionizing radiation are used not only in medicine, biology and biotechnology, but they are also widely used in industrial applications where they are particularly used as a means of non-destructive testing of any objects from the detection of defects in materials for quality control, to customs and police or security checks on delivered goods and transported objects.
The image of the scanned object is transmitted to a display area, either directly (projection display) or by data from the detector being computer-processed, the subsequent image created using the appropriate computer hardware and software.
The display area exists in many forms; the oldest type of display area is photosensitive film. With the advent of digitization, the most commonly used display areas are now scintillation screens (e.g. CsI, Gadox, NaI (TI), BGO, LYSO) in combination with photodetectors operating in the range of visible light (e.g. CCD or CMOS sensors). These systems utilize the principle of double conversion: radiation is first converted in the scintillator into visible light, then converted to electrical signal using a photodetector. The electrical signal is then processed using the appropriate hardware or software which creates an image on a screen or another medium.
In recent years, semiconductor detectors are becoming increasingly used as radiation detectors for imaging, operating on the principle of only a single conversion, when the impacting radiation generates an electrical signal directly in the semiconductor element. On one semiconductor chip, a large number of thus functioning elements (pixels) are created to form the image sensor. The signal from each element is further processed in specialized hardware and software that creates the final image. These detectors of radiation are known as semiconductor pixel detectors or sensors and are manufactured from different semiconductor materials such as silicon, CdTe, GaAs, etc.
Hardware for processing the electrical signals from each pixel is often formed on an independent chip, which is called an electronic reader chip, abbreviated as a reader chip. The chip of the semiconductor pixel sensor is usually located directly on the chip reader (overlapping) and is electrically connected to it with matrix contacts. Such an arrangement of both chips composes a non-separable unit which is referred to as a hybrid semiconductor pixel detector, or a hybrid detector, for short. The reader chip is fitted on at least one side with peripheral contacts used for power and communication with the hybrid detector. The area of the peripheral reading sensor is usually not covered by a pixel sensor chip, which allows for the connection of external conductors.
Examples of hybrid semiconductor hybrid detectors include the hybrid detectors Medipix2, Medipix3 and Timepix developed by the international collaboration of Medipix2 and Medipix3, or the hybrid detectors Pilatus and Eiger developed at the Paul Scherrer Institute. The thickness of the sensor layer is typically in the range of 50-2000 μm, wherein sensors for imaging are preferably used with a thickness of 300 μm and more. The sensors are mainly made from silicon crystal, less often from crystal CdTe or Cd(Zn)Te. Individual pixels are generally square shaped with a side of 55 μm (Medipix2, Medipix3, Timepix), 75 μm (Eiger), 172 μm (Pilatus), etc.
For hybrid semiconductor pixel detectors, the problem arises of creating a coherent image of a larger scanned object, because the maximum size of the detection area of one hybrid detector is limited by its maximum technologically achievable size. The typical technological limit in the production of reader chips is in units of cm2 (often shaped in a square with dimensions of 20×20 mm2). For larger areas, the probability of manufacturing defects is already high, and production becomes inefficient. The larger detection area must therefore be composed as a mosaic of several independent hybrid detectors. Each hybrid detector of this mosaic becomes one segment of a larger detection area. Hereinafter, the term “detector segment” will thus indicate a hybrid detector which is part of a larger detection area.
During the construction of larger detection areas to form a continuous image (larger detection area meaning that the area is larger than the area of one detector segment), systems are therefore used that enable the combination of detector segments into the detector surface with the mounting of individual detector segments into a flat matrix. The contacts of the periphery of the reader chip are connected to the bus in the flat matrix.
The problem which must be addressed in mosaic structure of the detection area is that the known solutions of detector segments use a sensor layer which have inactive (non-sensitive) edges and which therefore do not permit the detection of radiation and the design of an image from the entire area of the sensor layer. This gives rise to a so-called dead edge zone, or frames around the perimeter of individual detector segments that make it impossible to create a coherent image but divide the image into individual image segments from individual chips.
The solution according to international patent application WO 95/33332 to eliminate dead zones or frames from the inactive edges of the sensor layer of detector segments assembled into a mosaic consists in the mosaic containing vertically more layers of detector segments, the rows of which overlap each other so that the edge of the bottom row overlaps the edge of the upper row from the bottom. The detection area and the scanned subject move relative to each other at the same time, and using software for composing the display from individual detector segments in different positions of relative movement produces a continuous image of the scanned object with the elimination of dead zones. The disadvantage of this solution lies in the fact that the operation of the detector requires calibrated equipment for producing the relative movement and appropriate special software, so the manufacture and operation of such a detector is therefore complicated and costly. Overlapping insensitive edges also reduces the sensitivity of the detector in the marginal areas.
Other known solutions of mosaic detectors, e.g. according to the published patent application EP 0421 869 (A1) and US 2001/0012412 (A1), for the more efficient creation of mosaics and easier connection of the output wires from the periphery of individual detector segments to the bus at the bottom carrier matrix, use a tiered arrangement of the detector segments.
The detector segments, according to these documents, is configured such that the bottom reader chip on one side of a square or rectangular detector segment overlaps the upper sensor layer. This overlap forms a tier in which first, the output wires are led out from the reader chip to the bus, and secondly, that this tier in the composite mosaic is overlapped by the edge of the adjacent detector segment. The detector segments are stacked in the mosaic by individual lines, where the overlaps, respectively tiers, are oriented on one side of the tow, so the following row always overlaps the previous row. The tiered, respectively stepped, arrangement of rows in the detection area does not result in a defective image, since the tilt angle of the detector segments is minimal and can be easily corrected to create a digital image. Even here, however, due to the location of the insensitive material above the sensitive layer a reduction in sensitivity occurs in these areas.
According to document US 2011/0012412 (A1) a construction of detector is known in which the lower matrix is formed such that the rows of detector segments overlap stepwise or on a plane, while each detector segment is replaceable. To this end, each detector segment is fastened on a chip carrier which is mounted by a screw to the bottom of the matrix. The chip carrier may be flat or wedge-shaped, depending on whether the display area has to be stepped or substantially planar. Between the chip carrier and the lower matrix there is inserted an insulating layer with conductive contacts for each chip carrier. The chip carrier also serves as a bus for the output conductors of the reading chip, which lead into the chip carrier. The device according to US 2001/0012412 A1 partially eliminates the problem of dead zones in the creation of a coherent image, but only in areas in which the edges of the sensor layers of individual detector segments overlap, i.e. in lines of individual rows of the detector surface. Even though the segments are placed close side by side, their side edges, respectively the area along the side edges, still show inactive surfaces that form dead zones in the final image of the scanned object. These dead zones, in the creation of a coherent image, must be digitally blurred and overlapped, which, however, when used in medicine or flaw detection is a serious problem in terms of inaccuracies of the resulting image. The inactive edges of detector segments used in the solution according to US 2001/0012412 A1 arise in the manufacture of the sensor layers of the detector segments. The individual segments of the sensor layer are produced by cutting material from larger boards, which causes damage to the material in the area adjacent to the cut, so this narrow region along the edge, respectively sides, of a square or rectangular segment is inactive and cannot detect radiation. Solutions according to US 2001/0012412 A1 may eliminate some of the disadvantages of the solutions according to WO 95/33332, but they do not eliminate the fundamental problem of the existence of dead zones in the continuous image of the scanned object, so therefore a coherent and realistic image of the scanned object is not created by this solution.
Another disadvantage of the solution according to US 2001/0012412 A1 consists in that although the carriers of individual detector segments are fixed in the matrix of the detector surface removably, it is only in a specific and fixed position which does not allow for the positioning of detector segments to the side to limit their mutual lateral clearance. This creates gaps between the detector segments which form other dead zones that prevent the creation of a contiguous digital image.
The present invention is therefore to create such a detector of ionizing radiation which would eliminate the shortcomings of known solutions and would allow for the creation of a completely coherent and realistic digital image of the scanned object without the need for extra hardware or software resources to compensate for dead zones.